1. Field of the Invention
The present invention relates to methods of inspection useable in the manufacture of devices by lithographic techniques and to device inspection apparatus.
2. Description of the Related Art
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g., in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g., an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing,” Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
The inspection step after development of the resist, commonly referred to as metrology, serves two purposes. Firstly, it is desirable to detect any target areas where the pattern in the developed resist is faulty. If a sufficient number of dies are faulty, the wafer can be stripped of the patterned resist and re-exposed, hopefully correctly, rather than making the fault permanent by carrying out a process step, e.g. an etch, with a faulty pattern. Secondly, the measurements may allow errors in the lithographic apparatus, e.g. in illumination settings or exposure times, to be detected and corrected for subsequent exposures.
A metrology measurement may be used to determine the overlay error between two layers of a wafer, or may be used to determine focus errors or the critical dimension (CD) of features of a particular layer of the wafer (usually the uppermost layer). There are a variety of ways in which the metrology measurement may be obtained. Typically, these are performed in an apparatus which is separate from the lithographic apparatus. Measurements performed in a separate apparatus are commonly referred to as off-line. A single off-line apparatus may be used to perform metrology measurements for wafers produced by several lithographic apparatus.
One known off-line metrology apparatus, used to measure overlay is based upon imaging of boxes printed on the wafer, a first box being printed on a first layer and a second box being printed on a second layer. This apparatus may also be used to measure focus errors. The apparatus is commonly referred to as box-in-box (or frame-in-frame). A disadvantage of the box-in-box apparatus is that its general accuracy is limited because it relies upon single line image detection.
A second known off-line metrology apparatus comprises a scanning electron microscope (SEM). This provides very high resolution measurements of the surface of a wafer, and is used for CD measurements. A disadvantage of this apparatus is that it is slow and expensive.
A third known off-line metrology apparatus is known as a scatterometer. This provides measurement of CD and/or overlay. In a scatterometer, white light is reflected by periodic structures in the developed resist and the resulting reflection spectrum at a given angle detected. The structure giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled-Wave Analysis (RCWA) or by comparison to a library of spectra derived by simulation. However, the reconstruction of the structure is computationally very intensive and the technique can suffer from low sensitivity and poor repeatability.